Every enemy is a living barrel. There are four types: slime-filled, blood-filled, tar-filled, and crap-filled. They all act the same though. When the barrel knows you're there, it will go after you. As soon as it's next to you, it will blow up on its own. In levels with a lot of enemies, you will find yourself running for your life. NEVER run out of ammo, or you're doomed!